What pencil gesture always opens the large on screen keyboard?

You presumably refer to the iPadOS floating keyboard.

To easily return the new Floating Keyboard to the normal on-screen keyboard presentation, use the two-finger “zoom” gesture (i.e., the reverse of the two-finger “pinch”).

If the Split Keyboard is restored instead of the full on-screen keyboard, you can either use two fingers (one on each of the two half-keyboards) and swipe the two halves together - or touch-and-hold the “keyboard” key, then slide your finger to the Merge & Dock option and release.

Use the floating keyboard on your iPad – Apple Support

Split and move the keyboard on your iPad – Apple Support

Simply place your thumb and forefinger anywhere atop the mini software keyboard and spread those fingers outward to enlarge the keyboard back to its full screen software keyboard.

Best of Luck to You!